1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst system, and more particularly, to an exhaust gas-purifying catalyst system, which is excellent in the capability of eliminating nitrogen oxides in exhaust gases.
2. Description of the Related Art
In recent years, the regulation of exhaust gas of motor vehicles is being made stricter, increasing the necessity for further decreasing the amounts of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust gases.
Catalysts having a catalyst active component consisting of a noble metal such as platinum (Pt) carried on a porous carrier such as alumina, which in turn is supported on a heat resistant support structure such as cordierite have been widely used to purify the exhaust gas from a motor vehicle. The supported catalyst noted above is called a 3-way catalyst because the catalyst can oxidize CO and HC and reduce NOx in the exhaust gas simultaneously. In response to the requirement for stricter regulation of exhaust gas, the layer structure of the 3-way catalyst is being improved from the single layer structure to the double-layer structure or to the tandem type structure or the zone coat structure so as to improve the catalytic performance.
For example, Japanese Patent Disclosure (Kokai) No. 2-203938 discloses a catalyst system of a double-layer structure having a first activated alumina coat layer containing Pd, formed on the surface of a honeycomb structure, and a second activated alumina layer containing Rh, formed on the first layer, in order to improve the catalytic activity under low temperatures and to decrease the amount of platinum used as the noble metal active component. This prior art also teaches that the life of the catalyst system can be improved by adding traces of barium carbonate to each of the first activated alumina layer and the second activated alumina layer.
Further, Japanese Patent Disclosure No. 7-60117 discloses a catalyst system of a double-layer structure having a first alumina layer containing prescribed amounts of Ce, Zr and Pd, formed on the surface of a support, and a second alumina layer containing prescribed amounts of Pt, Rh, Ba and Ce, formed on the first layer, in order to improve the eliminating ability of, particularly, hydrocarbons (HC) in the exhaust gas and to improve the durability of the catalyst system at high temperatures. The barium (Ba) is supplied from an aqueous solution of a water-soluble barium compound such as barium nitrate or barium acetate.
However, the value of regulation in respect of NOx in the exhaust gas is rendered severer and severer. In this connection, it is of high importance nowadays to develop a catalyst system capable of more effectively eliminating NOx in the exhaust gas. It should also be noted that, in order to suppress the discharge of NOx contained in the exhaust gas, in driving the automobile, it has been attempted to employ a control system to control the air-fuel ratio to be from stoichiometric to somewhat rich. However, it has been clarified in recent years that with this control system, a larger amount of NOx is emitted in the exhaust gas on the contrary.